The sun and moon travel through the sky

March 8, 2011

The rapid lengthening of the days makes it clear that, however long the winters may seem in the Adirondacks, spring is inevitable.

On Sunday, March 20 at 7:21 p.m. EDT, the sun will pass over Earth's equator to shine again on the northern hemisphere after its southern winter journey. The sun doesn't actually move relative to Earth, of course, but Earth moves into the part of its orbit where the northern hemisphere is tilted toward the sun. If Earth's axis were perpendicular to its orbit, the sun would always shine on the equator. But the axis is 23.5 degrees shy of that, so the sun's path, the ecliptic, is tilted 23.5 degrees from the celestial equator, the projection of Earth's equator onto the sky.

If you were to stand on the east-west line marked at the Mitad del Mundo (middle of the world) in Quito, Ecuador (at zero degrees N), the celestial equator would arc up from the east point on the horizon, through the zenith directly overhead, to the west point. If you then started walking north, the plane of the celestial equator would stay attached to the east and west points on the horizon, but tilt southward at an angle equal to your latitude. If you were to stand on the north (or south) pole, the plane of the celestial equator would lie on the horizon.

In the Adirondacks, the plane of the celestial equator is tilted almost 45 degrees southward from the zenith. To trace it on the sky, scribe a circle from due east to due west that passes through Mintaka, the right-most star in the belt of Orion that's a quarter of a degree south of the celestial equator. This is a good night to do it since Mintaka will transit cross the north-south meridian and be at its highest point for the night at 6:24 p.m. Though the turning seasons will take Mintaka from the night sky, the celestial equator always remains in the same position. If Earth had rings like Saturn, we would always see them there.

The ecliptic crosses the celestial equator at the equinoxes. Jupiter has been close to the vernal equinox throughout the winter as the sun has been moving toward it along the ecliptic at about 1 degree per day. It is now so close that the vernal equinox hovers just above the western horizon at sunset for these last days of winter.

The diagram shows that Mercury has risen into the evening sky after swinging around the far side of the sun and will pass close to the vernal equinox tonight. Tomorrow night it will be slightly above the vernal equinox and half a degree above Uranus, which has moved a mere 3 degrees eastward since November, whereas Jupiter has moved 15-and-a-half degrees. (Your little finger held at arm's length is about 1 degree wide; your fist at arm's length is about 10 degrees.)

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If you have a low western horizon and good binoculars or a small telescope, Mercury can guide you to a last glimpse of Uranus before it is lost in the sun's glare. They set a mere hour after sunset, though, so the twilight will provide a challenge. On March 15, Mercury and Jupiter will be within 2 degrees. They'll only be 12 degrees above the horizon at sunset, but both are bright and set almost an hour-and-a-half after the sun. Thus, observers with low horizons have a good chance of spotting them.

A challenge many observers seek is the young moon, and this is the best time of year to see one. The new moon occurred on Friday, March 4 at 3:46 p.m. As it has continued on its orbit, it has moved to the east (left) of the sun into the evening sky. This motion slowly allows us to see more and more of the sunlit side of the moon. (To see how this works, face a bright light and move an orange around your head to the left. To see the "full orange," make sure your head doesn't eclipse the light!)

As shown in the diagram, the moon moves generally along the ecliptic. (Its orbit is tilted a little more than 5 degrees from Earth's.) In the spring, the ecliptic is at its steepest angle relative to the evening horizon, so the waxing moon's motion takes it rapidly away from the horizon. The moon must be at least 7 degrees from the sun for any of the sunlit side to be visible; thus, the youngest moon that could possibly be seen would be nearly half a day old. Indeed, Mohsen Mirsaeed of Tehran is credited with sighting a mere 11-hour, 40-minute-old moon with optical aid. The youngest moon seen with an unaided eye was 15 hours, 32 minutes old, as spotted by Stephen O'Meara in May 1990, according to Sky & Telescope. On Saturday, March 5, the 26-hour-old crescent was spotted in North America, according to moonsighting.com.

The sighting of the crescent moon marks the start of the month in the Islamic and Hebrew calendars. Though calculations of when it's visible are quite accurate, moon sighting is still an important activity. During a year I spent in Tucson, we had a group of 20 or so who would gather on a mountain pass west of the city to look for the young moon. When it was spotted, one man would greet it by blowing his shofar (ram's horn) to mark the new month.

The best chance for sighting a young moon will occur for us on April 4, after the new moon at 10:30 a.m. on April 3. Look for it around 7 p.m. (half an hour after sunset) a mere 10 degrees above the horizon. It's easier to see it first in binoculars, then with your eye.

Whether or not you're successful with the moon, turn to the east to spot Saturn, at opposition (opposite the sun in our sky), rising as the sun sets!

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If you have any questions about astronomy, visit the Adirondack Public Observatory website at apobservatory.org or e-mail Aileen at aodonoghue@stlawu.edu.